Thermodynamic and Kinetic Origins of Ferroelectricity in Fluorite Structure Oxides

Ferroelectricity in fluorite structure oxides such as HfO2 and ZrO2 has been intensively studied since the first report on it in 2011. The ferroelectricity in this material system is induced by the formation of a non-centrosymmetric orthorhombic phase, which is not thermodynamically stable under the normal thin-film processing conditions. Therefore, the thermodynamic and kinetic origins of the formation of the ferroelectric phase have yet to be clearly elucidated. Here, the previously proposed thermodynamic model based on the surface/interface/grain boundary energy argument is critically reviewed, and it is concluded that the thermodynamic model could not account for the emergence of the metastable phase accurately. Subsequently, the probable kinetic paths of phase evolution during the crystallization annealing and cooling processes are reassessed. A phase transition in Hf1-xZrxO2 thin films can be considered a feasible example of the well-known Ostwald's step rule involving a two-step phase transition with different kinetic energy barriers. It is concluded that the emergence of the ferroelectric orthorhombic phase can be mainly attributed to a kinetic origin: the transition to a stable nonferroelectric monoclinic phase either from the tetragonal or orthorhombic phase, which must be initially formed during the crystallization anneal, is kinetically suppressed.